1. Field of the Invention
The present invention relates to a composite structure with one or several polyurethane layers, a method for their manufacture and use of such structure, in particular as imitation leather.
2. Description of the Related Art
Composite structures are structures made of composite materials and obtained by combining different materials, with the chemical, physical and other properties of the composite structures being superior to those of the individual components. Composite materials include, among others, textile composites, fleece, laminated layered materials, i.e., materials having a sandwich structure (e.g., plywood, laminated foils and other laminates) and joined using an adhesive or a lining, as well as imitation leather. Imitation leather is to be understood as a multi-layer flexible composite material which includes a polymer in the surface layer and a support material which can, in particular, be made of textiles, fleece or an expanded material, such as PVC, polyolefine or polyurethane. The polymer cover layer protects against abrasion and impact and determines the visual appearance, whereas the support provides the mechanical strength and elastic properties. Polyurethane is also used as a polymer for coating. A distinction is generally made between single-component and two-component coating systems.
The single-component coating materials are processed in the form of solutions (solid content approximately 20 to 30%) in organic solvents (e.g., DMF, 2-propanol, toluene) or in form of dispersions (solid content approximately 20 to 40%). After spreading, e.g., onto a tape, as described in DE-A-4422871, the film is formed by evaporating the solvent in a drying channel. Single-component polyurethane compounds can be cross-linked after deposition by addition of slowly reacting poly-functional cross linking compounds (e.g., aliphatic poly-isocyanates) to improve their properties, for example, their resistance against chemicals. Due to their low solid content, single-component polyurethane coating systems can be applied as thin films.
Two-component coating systems are reactive mixtures made of, for example, functional pre-polymers and cross-linking compounds containing a small fraction of organic solvents (<5 to 10%). Unlike the single component systems, these “high solid systems” polymerize under processing conditions, thereby forming a urethane film. To provide an adequate pot life (defined as the time period during which a mixture, after all components are mixed together, can still be processed) at room temperature, isocyanate compounds are employed, with the isocyanate end groups being reversibly protected by blocking substances (e.g., 2-butanone-oxime). The chemical reaction proceeds here in two steps. Initially, at temperatures above 140° C., the blocking agent is split off and the free NCO-group is regenerated. In a second step, the isocyanate end group reacts with the chain extender by increasing the molecular weight. The molecular weight of the polymer thereby gradually increases until it reaches the molecular weight of the polyurethane film. Disadvantageously, the selection of blocking isocyanate derivates for this system is relatively limited. Hence, the selection of the educts is rather limited, making it difficult to achieve the desired properties of a product by changing its the composition. As a further disadvantage, the system can attain a high reaction rate only with a long residence time and at high temperatures. This limits the production speed and increases the production costs, which is partly due to the high price of the blocking isocyanate derivates.
It is therefore an object of the present invention to take advantage of the great variety of available and usable educts to produce composite structures with polyurethane layers over a wide range of compositions and a broad spectrum of characteristic properties. It is also a goal to lower the production costs. It is another object of the invention to provide a suitable method for manufacturing the composite structures.